Examples of freezing cycles in the prior art include the one disclosed in Japanese Unexamined Patent Publication No. H 7-25231. This freezing cycle, which uses a freon coolant, comprises, at least, a compressor which compresses the coolant, and a condenser, an expansion valve and an evaporator which are connected to the compressor in series. The freezing cycle is further provided with an auxiliary coolant passage set parallel to the expansion valve and a valve for opening/closing the auxiliary coolant passage, and the auxiliary coolant passage is opened when the low-level pressure becomes equal to or lower than a specific value.
In this example, by allowing the high pressure to bypass the expansion valve to directly flow into the low-pressure side and by thus preventing low-pressure from becoming reduced, increases in the pressure and the compression temperature at the outlet side of the compressor are prevented.
In addition, there are freezing cycles provided with a device for turning off the cycle by judging that the quantity of the coolant is not sufficient or that the load is too low due to a low external air temperature when the high pressure becomes equal to or lower than a specific value, i.e., freezing cycles provided with a low-level cut switch.
Other safety mechanisms that may be provided in freezing cycles include a device that stops the operation of the compressor when the high pressure becomes equal to or higher than a specific value, a device that stops the compressor when the outlet temperature at the compressor becomes equal to or higher than a specific value, a device that discharges the high-pressure coolant into the atmosphere when the high pressure becomes equal to or higher than a specific value and a fusible plug that allows the coolant to be discharged into the atmosphere when the coolant temperature becomes equal to or higher than a specific value.
However, while extensive research has been conducted into substances other than freon, such as carbon dioxide (CO.sub.2), to be used as a coolant in the freezing cycle in air-conditioning systems for vehicles to address the global environment issue, a freezing cycle using carbon dioxide, which has a low critical point of approximately 31.1.degree. C. as the coolant constitutes a reciprocal critical cycle that crosses over the critical point and, as a result, the high pressure in such a freezing cycle is as high as 10 times the high pressure of the freon coolant. Since this pressure level is close to the limit of tolerance of aluminum, it has become more crucial to provide an effective safety device for cycle protection than in a freezing cycle in the prior art that uses a freon coolant (conventional cycle) when designing a heat exchanger or the like by taking into consideration the safety factor.
In more specific terms, since the normal operating pressure on the high pressure side is approximately 10.about.15 MPa and the coolant does not cross over the critical point to become condensed in the reciprocal critical cycle, the high level side pressure reacts more sharply to fluctuations in the load compared to a freon coolant which becomes condensed on the high pressure side. Thus, it is understood that the likelihood of the high pressure in a reciprocal critical cycle reaching the vicinity of the maximum normal operating pressure is much higher than the likelihood of that occurring in the cycles currently in use. If a high-pressure cut switch is employed in a reciprocal critical cycle, a problem will occur in that the high-pressure cut switch will be activated too frequently. It is to be noted that the high pressure in the conventional cycle is approximately 1.2.about.3 MPa, with the setting for the high pressure cut switch at approximately 3 MPa.
In addition, since the critical point of the coolant is low in the reciprocal critical cycle, the balance pressure between the high-pressure side and the low-pressure side in the reciprocal critical cycle left in hot daylight becomes as high as approximately 10 MPa. While it is desirable to minimize the coolant volume on the high-pressure side in order to prevent the balance pressure from rising, there is a problem in that minimizing the coolant volume on the high-pressure side would increase the passage resistance on the high-pressure side to result in a large pressure loss.
Thus, when using carbon dioxide as an alternative coolant to freon, it is necessary to take safety measures to prevent damage to various components which would be caused by the high pressure while taking into consideration the eventuality of such damage occurring.
Accordingly, an object of the present invention is to provide a freezing cycle having a means for safety that is most suited for application in a freezing cycle using carbon dioxide as a coolant.